1. Field of the Invention
The present invention relates to a multi-channel or parallel electro-optical module and method of operation thereof wherein a loss of signal can be detected at a receive end of a parallel optical link, and information relating thereto communicated to a transmit end.
2. Description of the Related Art
A typical parallel optical transceiver consists of a vertical cavity surface emitter laser (VCSEL) array, to provide N optical beams coupled to N parallel optical transmission channels or lanes, and a photodiode array, to provide N parallel optical receiver lanes. A parallel optical fiber ribbon connects the transmitter array of the first optical transceiver, with the photodiode receiver array of a second optical transceiver, and vice versa.
Various standards bodies typically dictate that in the case of a fault condition where laser light could be exposed, an optical transceiver should enter an “eyesafe” mode, wherein the average optical power transmitted by the transceiver is reduced to a level which will not cause damage to the human eye. In the case of parallel optical transceivers, meeting the “eyesafe” standards can be difficult, because of the parallel nature of the arrangement. In this respect, a parallel optical transceiver with N optical transmit and receive lanes, will typically transmit N times the power of a single optical fiber. Thus, if a parallel optical ribbon is damaged, a significant amount of laser light well above the “eyesafe” levels can be released.
The problem is exacerbated for integrated cable/transceivers, where the parallel ribbon fiber is permanently attached to the electro-optical transceivers at each end. In this case, there is no accessible optical interface normally, and the optical power launched into the fibers is impossible to measure. Moreover, since the optical interface is not accessible, there is an unknown coupling loss in assembly, and hence it is not desirable to limit the maximum permitted power from each of the VCSELs. Thus, individual VCSEL optical emission may, in some cases, exceed the eye safety limit if exposed. In addition the power launched into a single fiber might exceed safe level if exposed due to fiber breakage.
US 2003/0095303 describes how open fiber control may be performed in an optical transceiver. More particularly, during normal operation a transceiver transmits signals through a connection to an optical network. When the connection breaks, the transceiver detects the loss of signal, and disables transmissions over all channels except for one. The transceiver continues transmission on a single enabled channel at an eye safe level. When the connection is fixed and the signal reappears, the transceiver detects the signal reappearance, and re-enables all channels that had previously been disabled.
A similar system is described in US 2002/0149810. Here, a duplex parallel optical link includes a transmitter and receiver pair and a fiber-optic ribbon that includes a designated number N of channels that cannot be split. The duplex transceiver includes a corresponding transmitter and receiver which are physically attached to each other, and cannot be detached therefrom, so as to ensure safe, laser optical power in the event that the fiber-optic ribbon cable is broken or severed. In the case of the fiber optic ribbon cable being broken or severed, all of the channels except for a designated safety channel are shut down. That is, only the designated safety channel remains enabled.
Within the prior art, therefore, the problem of maintaining eye safety in parallel optical transceivers is solved by shutting down all of the parallel optical lanes between two optical transceivers in the event of a fault being detected in one of them. Then, a single designated safety channel is maintained open, which transmits at an eye safe level, and which is used to detect when the link is repaired. However, this means that a fault in a single one of the optical fibers results in the entire link being lost. It would therefore be advantageous to provide a technique that a break in a single one of the optical fibers does not mean that the entire parallel link is lost, and some of the optical lanes can remain open.